Investigation into the hydrothermal humic acid production from the perspective of lignocellulosic biomass model components

The regulation of humic acid yield, properties, and composition is paramount for its high-value transformation. Delving into the formation mechanisms of humic acid holds significant implications for manipulating its structure and properties, as well as preventing the formation of non-humic acid byproducts during conversion processes. In this study, lignocellulosic model components, including cellulose, hemicellulose, lignin, pectin, proteins, and lipids, were hydrothermally converted into humic acid. Characterization methods including FTIR (Fourier Transform Infrared Spectroscopy), EA (Elemental Analysis), and SEM (Scanning Electron Microscopy) were used to study the effects of blending on humic acid's properties and component interactions. The hydrothermal conversion route with the highest yield of humic acid was evaluated by the life cycle analysis. Findings show that cellulose, hemicellulose, and lignin are key to humic acid formation. Cellulose and hemicellulose contribute more to the yield than lignin, but lignin is the backbone. Pectin reduces the yield as its methyl ester groups hydrolyze, competing for reactive sites. Lipids, especially oil, alter humic acid's composition and structure, and adding oil can decrease humic acid production. Lignin increases the O-C=O valence state and oxygen content, while oil affects the C-O-C valence state and element ratios. Humic acid has good thermal stability due to components like lignin and proteins. The hydrothermal reaction stage, characterized by the concentration of electrical and thermal energy, has the most significant environmental impact. During industrial-scale amplification, this process exacerbates soil acidification, increases CO2 emissions, consumes both fossil and renewable energy resources, and elevates PM 2.5 emissions. However, industrial-scale processes mitigate eutrophication and reduce water waste. This study clarifies the roles of biomass components in humic acid formation, offering insights into lignocellulose hydrothermal conversion and a basis for future research in this area.